High frequency band filter



April 9, 1935. K. Pos'rHuMus ET AL 1,995,331

7 HIGH FREQUENCY BAND FILTER I Filed Jan. 2, 1952 INVENTORS KLAAS POSTHUMUS BY ;%R J.WEYER5 MVM/ ATTORNEY Patented Apr. 9, 1935 Delaware ,P osthumus land weyers Eindhoven, Netherlandmassignorsl'to V Radio Corporation'ofAmerlca,acorporationoff,

1 Claim... (01.178-44) In many cases, for instance, in wireless teleg- 5, cies, both the higher and the .lowerfrequencies,

are suppressed as much aspossible. In apparatus constructed for receiving radio waves it is desirable, moreover, tobe able to adjust the average height of the frequencyjband which is allowed to pass to the carrier wave frequencies of all of the transmitting stations to be received.

According to the present invention this purpose is attained by capacitively coupling two or more tunable oscillatory circuits'by means ,of one or more condensers of such a size that the'coupling in or near the middle of the covered frequency range is critical. By critical coupling is to be understood in this case a coupling at which the crest of the resulting resonance curve of the coupled circuits begins tosag. I

The novel features which we believe to be characteristicof our. inventionare set forth in particularity in the appended claim, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in,-

connection with the drawing in which wehave indicated diagrammatically one circuit arrangement whereby our invention may be carried into effect.

The invention will be more clearly understood I by reference to the accompanying drawing, representing by wayof example, a form of construction thereof used in the antenna circuit of a receiving set.

The antenna l is connected to earth through an antenna condenser 2 and a coil 3. A tuning condenser 4 anda fixed coupling condenser 5 are connected in parallel to the coil 3. The condenser 5 valve of the screen grid type.

The condenser 5 is given such a size that. if

- both condensers 4 and 6 occupy'almost their cen- The coupling between the two tuned circuits is capacitive, thus by increasing frequency the coupling decreases. The value of the coupling capacity is thus chosen, that forthe frequency'at the middle of the frequency range, the coupling is critical. I V

i f: IlIGH FREQUIlNCY" 3m 2 What is meant by critical coupling :may be declared as follows:

- If two tuned circuits are coupled very loosely, v v

the resultant characteristic ofthe' system shows onepeak, the height of which increases with increasing coupling un'til the critical coupling is reached, a'tighter coupling than this .critical coupling" causes a resonance curve with two peaks, the height of which is about constant and l the distance between the peaks being, greater for tighter coupling of the circuits. a

The critical coupling of two given circuits is thus a definite coupling and when thisv coupling the magnitude of the impedances which the an. I

cuits contain.

The-advantage of using; a capacitive coupling is the following:

If the coupling is increased above critical the peaks of the resonance curve are more separated,

but at the same time the slope of the sides of the resonance curve is steeper (because each of'the resonance curves of the separate circuits is narrower for lower frequencies and for these the coupling of the circuits is tighter). This results in a resonance curve which has equal width for different frequencies. Q

The best results are obtained when using a capacitive coupling which is critical in the middleof the frequency range. If the coupling would be made critical for one of the lowest frequencies to be transmitted, the coupling for the highest 1 40 frequencies would be so small thatthe energy transfer is too much decreased.

If on the other hand the coupling is madecritical forone of the highest frequencies, thecoupling for the lowest frequencies is so tight that the 45 resonance curve shows a deep sag in the middle, whichshould also be avoided. a

If the coupling is critical in the middle of the 1 5 frequency range, a frequency band of practically constant width may be transmitted with equal strength, neither a too loose, nor a'too tight couplingwith its inherent disadvantages ever being obtained. v 1

While we have indicated and described one Sysvtem for carrying our invention into eifect,'it ,will

be apparent to one skilled in the art that our ini am,

vention is by no means limited to the particular organization shown and described, but that many modifications may be employed, without depart ing from the scope of our invention as set forth in the appended claim.

Whatis claimedis: k

In combination-,2 in 'ai;v radio receiver, a screen grid amplifier tube," asignal circuit'adapted to have impressed thereon side band modulated car; rier frequencies of a predetermined frequency range, a tunable band pass network coupling-said g circuit to the input electrodes of the tube, said network comprisinga pair of oscillation circuits each including a coil andja variable tuningjcon f denser, a coupling condenser comn'i'onto'both'oscillation circuits, said signal circuit being connected across .'one of said coilsandsaidiinput electrodes being connected across the other coilf' quency bandwhich is transmitted through the network is ,adjusted to the carrier frequencies'of said range; I

I f KLAASLPOS'IHUMUS.

THEODORUS JOSEPI-IUS WEYERS. 

